feat: `stat` subcommand

[massivebird]

Assimilate arcstat into arcsearch! Is this a good idea?

Arcconfig was born because I wanted to build two separate utilities. If they become a single tool, then there's no need for anything like arcconfig. Bsrc already knows how to deserialize everything into data structures!

Extending "stats" with filters and keywords is similar enough to the "searching" that it wouldn't seem out of place as a subcommand.